Compared to other printing methods, an inkjet recording method offers ease of full-color printing since it involves a simpler process. The inkjet recording method provides high-resolution images in spite of being simplier.
As inkjet recording inks, dye-based inks are used in which various types of water-soluble dyes are dissolved in water or a mixture of water and an organic solvent. Such dye-based inks have poor light resistance, though they are excellent in color tone sharpness.
Pigment-based inks in which carbon black and various types of organic pigments are dispersed are superior in light resistance compared to that of dye-based inks, and thus studies on pigment-based inks are being intensively conducted.
However, pigment-based inks are more likely to cause nozzle clogging compared to dye-based inks.
Pigment inks are generally prepared as follows. A coloring material and a dispersant are preliminarily dispersed in an aqueous solvent to prepare a dispersion product. The dispersion product is then dispersed to a predetermined degree in a media-type disperser, followed by diluting to a predetermined concentration.
Water-based pigment inks usually contain a surfactant and a water-soluble resin to disperse a hydrophobic pigment therein. However, the majotiry of the pigment does not penetrate into an ink absorbing layer of the inkjet recording sheet and stays on the sheet to form a coating film. Therefore, when a water-based pigment ink is used for the inkjet recording sheet, a printed image may have lesser gloss or lesser abrasion-resistance than that of the aqueous pigment ink when used for plain paper or dye ink which penetrates into the ink absorbing layer easily. In this case, the printed image may have poor quality due to peeling of the coating film which causes ambient defacement, when the printed image is scratched.
Thus, according to some known techniques, film-formable resin fine particles are added into inks to improve quality of images. However, it is difficult to keep a plurality of components finely and stably dispersed throughout the ink for a long period of time. Thus, when a dispersant such as a surfactant is used in a large amount in order to stably disperse such fine particles, air bubbles may be generated inside the ink tank and/or inkjet head, which could cause degradation of the image quality.
Likewise, to improve dispersibility, the effectiveness of methods that change the surface of a pigment to a hydrophilic group, or, the effectiveness of using a resin containing a hydrophilic group has been studied. However, while a given component may be stable when used alone, the combined use of several different components may cause degradation of dispersion stability and storage stability.
Patent Literature (PTL) 1 to 3, Japanese Patent Application Laid-Open (JP-A) No. 2007-186642, JP-A No. 2006-282781, and JP-A No. 2005-105227, proposed a surface-treated pigment ink, which gives excellent printing quality, and has excellent jetting stability and storage stability, or has only excellent jetting stability. PTL 4, JP-A No. 2007-119551, proposed an ink set, which can maintain stable jetting performance of inks.
To obtain a high image density of a printed image, PTL 5, JP-A No. 2006-8858, discloses a method in which ink containing a water-insoluble coloring agent and charged resin quasi-particles which are smaller than particles of the coloring agent, is used. PTL 6, JP-A No. 2002-3767, discloses a method in which ink containing a self-dispersible pigment having an optimized DBP oil absorption is used; and PTL 7, JP-A No. 2006-219584, discloses a method in which an aqueous dispersion liquid of carbon black containing carbon black which has been surface-modified to have the HLB value of 7 to 18, and a nonionic surfactant having an acetylene skeleton in the molecular structure thereof, is used.
PTL 8 JP-A No. 2004-123904, discloses aqueous ink containing an acetylene glycol-based surfactant.
To stabilize a dispersion state of an ink, PTL 9, JP-A No. 05-239392, discloses a method of dispersing a water-dispersible resin having a carboxyl group and a nonionic hydrophilic group in the molecule thereof in water; PTL 10, JP-A No. 08-283633 discloses a method in which polarities of a water-soluble polymer and a surfactant are made the same, or nonions are added thereto; PTL 11, JP-A No. 2000-63727, discloses a method in which an aqueous recording liquid is controlled so that an ionic coloring agent-containing a polyester resin and hydrophilic groups of the coloring agent have the same polarity; and PTL 12, JP-A No. 2001-81366, discloses a method in which a dispersed pigment and dispersed resin particles are controlled to have the same polarity.
PTL 13, JP-A No. 2003-509571, discloses a printing ink using a gemini surfactant as a dispersing agent.
PTL 14, JP-A No. 08-333531, describes an aqueous inkjet ink composition containing a pigment dispersion liquid, an aldehyde naphthalene sulfonate dispersing agent, and/or at least one sulfone solvent, wherein the pigment particle dispersion liquid contains pigment particles having the particle size distribution so that at least 70% of the particles in the dispersion liquid have a diameter of less than 0.1 μm, and the other particles have a diameter equal to or more than 0.1 μm. PTL 15, JP-A No. 56-147871, describes a recording liquid formed of an aqueous medium containing a pigment, a polymeric dispersing agent and a nonionic surfactant.
PTLs 16 and 17, U.S. Pat. No. 5,085,698 and U.S. Pat. No. 5,221,334, describe the use of an AB or BAB block-copolymer as a dispersing agent of a pigment. PTL 18, U.S. Pat. No. 5,172,133, describes that a specific pigment, water-soluble resin and solvent are used.
As a method for dispersing a pigment without using a dispersing agent, PTL 19, U.S. Pat. No. 5,571,311, discloses a method in which a substituent including a water-soluble group is introduced into carbon black; PTL 20, JP-A No. 08-81646, discloses a method in which a water-soluble monomer is polymerized onto a surface of carbon black; and PTL 21, JP-A No. 08-3498, discloses a method for an acid-treated carbon black.
PTL 22, JP-A No. 09-194775, describes a method in which water-resistance and jetting stability are attained by using an ink containing an acid-treated carbon black, and a terpolymer formed of acrylic acid, styrene, and α-methyl styrene.
PTL 23, JP-A No. 2000-144028, describes an inkjet recording liquid containing dispersed particles having the average particle diameter of from 30 nm to 200 nm.
Although conventional ink liquids could provide a high image density with a color pigment ink, a black pigment ink does not provide a sufficient image density, and satisfactory black pigment inks have not yet been described.
PTL 24 to 26, JP-A No. 2005-281691, JP-A No. 2005-314528, and JP-A No. 2006-188626, describe examples in which beads having a diameter of approximately 0.05 mm to approximately 1.0 mm are used for the bead mill dispersion; however dispersion stability of the resultant dispersion is not sufficient.
PTL 27, Japanese patent (JP-B) No. 3625595, describes the use of an anionic surfactant as a dispersing agent, and that the molecular weight of a such surfactant is preferably in the range of 1,000≦m≦30,000. However, the resultant ink liquid does not have sufficient dispersion stability, and also loses its dispersion stability when a pigment that is not resistant to the impact caused by dispersing is used, and, as a result, the ink liquid has poor jetting stability.
PTL 28, JP-A No. 2011-116857, discloses a method for improving jetting stability and preservation stability by mixing two different types of dispersion elements, including a surfactant and a capsule, wherein a particle diameter of the mixture of elements is adjusted properly. However, in this method the bleeding can not be prevented.
To prevent bleeding of pigment ink, PTL 29, JP-A No. 2001-294788, discloses a technology for prevention of the movement of carbon black as a pigment, by agglutinating an element of black ink using an agglomerating agent, wherein the agglomerating agent is included in color inks such as cyan ink, magenta ink, and yellow ink. When using this technology, it is estimated that the bleeding of black ink may be prevented to some degree. This technology assumes that paper is a recording medium; however, the bleeding may not be prevented when this method is used on cloth having large spans between fibers compare to paper. In addition, because it is possible to wash cloth, the image dencity may become lower when cloth is washed and then ink is removed.
To prevent bleeding, PTL 30, JP-A No. 2011-6672, discloses a mixture containing two anionic self-emulsification ether-based polyurethane resins having different acid values. However, the bleeding can not be prevented sufficiently by this method.